The present invention is in the field of AC power systems and, more particularly, the present invention relates to adjustable frequency power generators, for example, of a type suitable for driving ultrasonic transducers.
In the prior art, power ultrasonic generators for driving resonant loads have principally been of the self resonant type. In such generators, a feedback signal from the resonant load to the power driver, or oscillator, automatically sets the drive frequency to the electrical resonant frequency (or to one of the electrical resonant frequencies when more than one exists). The desired drive frequency for the transducer is obtained by designing the electrical resonance of the output resonant circuit to have its primary resonant frequency equal to the desired drive frequency. Examples of this type of generator are found in U.S. Pat. Nos. 3,651,352 and 3,681,626. With the above type of circuits, if precise control of the drive frequency is desired, a method to change the output circuit resonant frequency must be incorporated; this is generally expensive and the response to changes is slow. Further, for generators without drive frequency control, when the environmental conditions (e.g. temperature changes) cause characteristics of the resonant components to change, the drive frequency also changes. This can cause the drive frequency to move out of its acceptable operating range for the transducer.
In the prior art, the Class A SCR inverter circuit (for example, as set forth in GE SCR Manual, Sixth Edition, 1979, pages 354 to 356) overcomes certain of these disadvantages. That circuit automatically matches the resonant frequency of the output to the drive frequency by using a single inductor or two inductors in parallel, for the required amount of time during each cycle. For example, when a higher drive frequency is employed, the time that the two inductors are in parallel is increased, thereby raising the equivalent electrical resonant frequency. The primary disadvantage of this circuit for power ultrasonic generators is that stored output reactive energy is returned to the input power supply during the time that the two inductors are in parallel. This extra flow of energy from output to input then back to the output necessitates higher current rated power semiconductor devices, thereby adding cost to the system.
An alternate approach to the problem of controlling the power drive frequency to reactive loads, is to not make the load resonant, but to drive the reactance directly and dump the stored reactive energy each half cycle. However, this approach is characterized by high inefficiency. As a consequence, this approach is impractical for all but the lowest power ultrasonic generators.
It is an object of the present invention to provide an improved AC power system for driving resonant loads.
Another object is to provide an improved AC power system characterized by high efficiency.
Yet another object is to provide an improved AC power system which automatically compensates for different drive frequencies and varying electrical resonant frequencies, without the need for adjustable reactive components in the output network.
Still another object is to provide an improved AC power system which transfers energy substantially only from the power supply to the load throughout each cycle of operation.